Then that's dropsy, and it's best if you separate her completely from her tankmates. There's not much you can do to save her, if anything, and she's probably so far along that she'll die shortly.
Dropsy isn't a SPECIFIC disease, exactly, but is the common name for a cluster of problems that may have their roots in bacterial infection or other problems.
If you want to try treating her, add some Maracyn 2 and try that. Also, if you can get your hands on Colloidal Silver, that MAY help. But, really, by this stage, her death is quite imminent, and often the most you can do is make her comfortable or euthanize her.